The present invention relates to sulfidation (hot corrosion) and deposit formation is gas turbine engines and more particularly to a method of inhibiting such sulfidation and modifying the character of such deposits.
It has been established that sodium sulfate is the agent principally responsible for hot corrosion attack in hot post-flame regions of gas turbine engines. This hot corrosion can severly damage gas turbines and be quite costly.
There are many routes by which sodium compounds can enter the gas paths of a gas turbine or other engine. For example, such compounds may be ingested as a component of a sea water spray (NaC1), as air-borne soil dust, as a deicing chemical, as a fuel (for example, as oil-soluble sodium salts of naphthenic acids), etc. Sulfur is a common constituent of fossil fuels, and even the most highly refined fuels contain some sulfur. Sulfur forms oxides on burning, and these oxides react with the aforementioned sodium compounds under engine operating conditions to give, as one product which is stable at high temperatures, sodium sulfate. Whether the sodium sulfate is formed in the flame, in the hot gas paths, on the alloy surfaces, or by any combination of these processes is immaterial, since its presence on the alloy at appropriate temperature is all that is required for hot corrosion.
Gas turbine parts, especially early-stage nozzles and blades, are subjected to gas temperature sometime approaching 1100.degree. C. These temperatures far exceed the fusion temperatures of pure sodium sulfate (884.degree. C.) and mixtures of sodium sulfate with other salts. Thus, the presence of sodium sulfate thereon, however derived, results in formation of a liquid film of sodium sulfate, or molten mixtures which contain sodium sulfate, on nozzles and blades. This molten salt, and/or its mixtures, are particularly corrosive to the metals of the gas turbine engine, whether they be bare metal, metal protected by a film of its own high-temperature oxidation products, or metal protected with a film of more corrosion resistant material (such as inert aluminum oxide).
Although the process may be complex, it is known that liquid sodium sulfate, by a combination of physical and/or chemical reactions, generates rapid self-sustaining hot corrosion of the metal parts. The molten salt attacks any protective oxide coating present on the metal surface, thus exposing the underlying substrate to accelerated corrosion. The presence of metal sulfides as corrosion products, along with metal oxides, leads to use of the term "sulfidation corrosion" to describe such sodium sulfate-induced corrosion although other terms such as "hot corrosion" or simply "sulfidation" are also used. This sulfidation can increase downtime, repair expense, and loss of generating capacity, and in extreme cases it can necessitate a major overhaul to replace severely damaged internal parts, at great cost to the engine operator.
For the purposes of this invention, it is instructive to differentiate between this sulfidation corrosion and sulfuric acid corrosion. Sulfuric acid corrosion is caused by condensation of sulfuric acid or sulfur trioxide with water on relatively cool portions of boilers, in the temperature range below about 200.degree. C. This is also called "cold-end corrosion", since it occurs in sections of boilers known as the cold end, or those portions where the flue gas has been cooled below the dew point of the sulfuric acid. In this cold-end corrosion, acidic conditions are required, and the process is essentially that of corrosion of metals by warm acid. Generally, cold-end corrosion becomes more severe as gas and/or metal temperatures decrease, at least down to a certain temperature, because more sulfuric acid condenses as the temperatures drop further below the dew point. Anti-corrodents for cold-end corrosion act through an acid neutralization reaction, whereby the anti-corrodents and/or their combustion products neutralize the sulfuric acid to form solid sulfate salts which are not corrosive at these cold-end temperatures because they are solid.
In contrast, the hot corrosion (sulfidation) which is the subject of this invention does not necessarily involve acids. The temperatures at which it occurs are far above those at which sulfuric acid condenses. In fact, sulfidation occurs only in the hottest portions of the engines, in contrast to sulfuric acid corrosion which requires condensation of sulfuric acid and thus occurs only in the color sections of boiler units. Sulfidation requires a temperature above the fusion point of the sodium sulfate (884.degree. C) and its mixtures, since solid salts have little or no corrosive effects in the hot portions of gas turbines. As acidic conditions are not required for sulfidation, anti-corrodents for cold-end corrosion are not necessarily of value as anti-corrodents for sulfidation.
In addition to the sulfidation problem discussed above, gas turbines are also subject to the problem of the adherence of deposits to the same turbine parts. Such deposits may consist only of naturally occurring impurities in the fuel and/or air, or may also include materials traceable to an additive treatment (such as a treatment to reduce sulfidation). These problems may or may not be separate ones. Either one can occur without the other, but interrelationships may also exist. Other problems may also aggravate either or both of these problems. For example, sticky soot (carbon or char) particles are formed by the incomplete combustion of fuel droplets. The sulfur oxides and nonvolatile metal-containing impurities (such as sodium chloride) concentrate on these chars. The chars eventually hit the hot parts of the turbine and stick thereto, thereby holding sodium sulfate or its mixtures in place to melt and then corrode the turbine hot parts. Carbon on the blades can also enhance the corrosivity of sodium sulfate, possibly by acting as a reducing agent to help convert sulfates to sulfides and oxides of the metallic alloy components. In these cases, the carbon is not necessary for corrosion or deposition, but it can enhance these processes and make their effects more severe.
As a practical matter, the two problems (i.e., sulfidation and deposit adherence) are closely intertwined from the point of view of solutions to the problems. Publications (such as that of R.M. Junge, "General Electric Company Experience With Oil Fired Gas Turbines", ASTM Symposium, Atlantic City, Jan. 28, 1964) and patents (U.S. Pat. No. 3,581,491) have described the use of chromium compounds as a sulfidation anti-corrodent. However, while tests reported by General Electric Company showed that sulfidation was controlled, the publication concluded that "the treatment was not useful . . . because it produced an extremely hard and rapidly accumulating deposit" (page 6). Furthermore, the recent Westinghouse Electric Corporation Liquid Fuels Specification MS-576010, Jan. 6, 1975, for gas turbine engines concludes with respect to sulfidation that "for gas turbines operating at gas inlet temperatures above 1200.degree. F, no additive has been found which successfully controls such corrosion without at the same time forming tenacious deposits". Thus, no anti-corrodent for sulfidation is known which does not result in unacceptable deposit formation.
Accordingly, it is an object of the present invention to provide a method of inhibiting sulfidation without introducing unacceptable deposit formation.
It is another object to provide a method of modifying deposit characteristics without increasing sulfidation attack.
It is a further object to provide a method of both inhibiting sulfidation and desirably modifying deposit characteristics.
One object is to provide such a method which enhances the efficacy of a known sulfidation inhibitor.
Another object is to provide such a method which not only enhances the efficiency of a known sulfidation inhibitor, but renders its use acceptable by modifying the character of the deposits resulting from its use.